According to regulations specified by countries and local telecommunication management departments, the telecommunication network should offer emergency voice call service. When users call the emergency call number, use default emergency call number saved in terminal equipment, or use emergency call identification to initiate an emergency call, the network will send the emergency call to public safety answering centre, such as police, fire police, ambulance, rescue, and so on.
Internet Protocol (IP) Multimedia Core Network Subsystem (IMS) also should provide emergency call service.
FIG. 1 is a reference framework of the IMS emergency call system. The whole framework comprises User Equipment (UE) 101, Proxy Call Session Control Function (P-CSCF) 102, Serving Call Session Control Function (S-CSCF) 103, Home Subscriber Server (HSS) 104, Emergency Call Session Control Function (E-CSCF) 105, Location Retrieval Function (LRF) 106 and Public Safety Answering Point (PSAP) 107, also it involves function entities (not shown in FIG. 1) such as Interconnection Border Control Function (IBCF) and Interrogating Call Session Control Function (I-CSCF) during the call.
In the method, E-CSCF 105 is a new function entity introduced into the IMS domain, it is in the same network (the visited network in which the user is roaming) as the P-CSCF is, and is responsible for receiving emergency call establishment request from P-CSCF 102, and routes the emergency call establishment request to PSAP 107 connected to the IP network through the Mm interface, or to PSAP 107 connected to the PSTN (Public Switched Telephone Network) through the Mi/Mg interface according to location information offered by the UE or location information and routing information obtained from LRF 106. Meanwhile, PSAP 107 can directly obtain the location information of the UE from the IRF 106 through Le interface, and communicate with its home network through Mm/Mw interface.
According to the service requirement of the IMS emergency call service, if the UE has enough authentication parameters and can detect emergency calls, the UE should use emergency public user identification to perform the process of IMS emergency call registration to home network through the P-CSCF of the visited network. This process is similar to the process of normal IMS registration. During the registration, the registration expiration time should be determined. For a normal IMS registration, the UE should initiate the procedure of re-registration in the registration expiration time period, renew the registration expiration time timer, and notify the network that the UE is in the registered status. If the registration timer is expired, the network (including P-CSCF and S-CSCF) and the UE will perform the process of de-registration to release the information related to the registration and the corresponding call. For emergency registration, the UE is not allowed to initiate the process of emergency re-registration, and if the registration timer is expired, the network (including P-CSCF and S-CSCF) and the UE will release the emergency registration. Only there is an emergency call going on, will the UE be allowed to initiate the process of emergency call re-registration.
In the related art, the process of emergency registration and the process of registration expiration time are shown in FIG. 2, which comprise the following steps:
201: When the UE detects that the call is an emergency call and it has enough authentication parameters, it uses the emergency call public user identification to send an emergency registration request including the registration expiration time, for example, the registration expiration time is set as 600000 seconds, to the P-CSCF;
202: When the P-CSCF receives the emergency registration request, it does not process the registration expiration time, and meanwhile forwards the request to the I-CSCF of the access point of the home network of the UE;
203: the I-CSCF sends S-CSCF assignment request message to the HSS to ask for S-CSCF assignment;
204: the HSS returns the S-CSCF assignment response message including the assigned S-CSCF address or S-CSCF capabilities to the I-CSCF;
205: the I-CSCF sends the emergency registration request message to the selected S-CSCF;
206: the S-CSCF sends user profile data request to the HSS;
207: the HSS saves the information of the S-CSCF, and returns the profile data and security information of the user to the S-CSCF by sending the user profile response message;
208: the S-CSCF processes the emergency call registration service logic, and decides the emergency registration expiration time according to policy of the home network;
Wherein the policy of the home network only considers the characteristic of the home network, but does not consider the call type, the requirement of the visited network, and so on, for example, the registration expiration time is uniformed regardless of the location of the user.
209: the S-CSCF constructs the emergency call registration response including the emergency call registration expiration time decided in step 208, and sends the response to the I-CSCF;
210: the I-CSCF forwards the emergency registration response to the P-CSCF;
211: the P-CSCF obtains the emergency registration expiration time determined by the S-CSCF from the emergency registration response;
212: the P-CSCF forwards the emergency registration response to the UE;
213: the UE obtains the emergency registration expiration time from the emergency registration response.
From the above process, it can be seen that the emergency call registration expiration time is decided by the S-CSCF of the home network according to the policy of the home network which does not consider the requirements of the visited network to the emergency registration expiration time. When the registration is expired, the S-CSCF, P-CSCF and UE on the session path will respectively perform the process of emergency call de-registration to release the local registration information.
According to the requirement of emergency call service, the emergency call service is serviced by the visited network, and after successful registration, the subsequent emergency session is controlled by the visited network, and the home network will not participate in the subsequent session.
Due to different regulations in the telecommunication management departments of the visited networks, each visited network has different time requirements for the registration expiration. For example, for emergency call of rescue, the time set in place A is half an hour, while place B is one hour, which requires the network and the user terminal to sustain the status of registration for a certain time after the emergency registration in order to meet the requirements of calling back, searching and rescuing. When a user in place A roaming to place B initiates an emergency call, the emergency call registration time will still use the setting of his home place according to the related art, that is, the expiration time is half an hour, while in practice, the registration expiration time required in place B is one hour, and thus it will not meet the requirement of emergency call service in place B.
Moreover, since the registration expiration time set by each home network is different, there is possibility that in the same visited network, the roaming users of different home networks and the local users have different registration expiration times, which will make the emergency call service characteristic inconsistent, thus making the application and management of the emergency call service more complicated.
From the above description, it can be seen that the present IMS emergency call registration has defects in processing the registration expiration time. Since the method of the home network setting the registration expiration time will make the registration expiration times of the users of different home networks different in one network, the requirements of emergency call service of the visited network may not be satisfied, and the consistence of the emergency call service characteristic will be affected, thus having the difficulty in the application and management of the emergency call service.